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The CBL Index is a ratio between the number of
IP addresses in a given IP subnet (Subnetwork) to the number of CBL (Composite Blocking List)
listings in the subnet. It may be used to measure how "clean" (of
compromised computers) a given subnet is.

The higher the number is, the "cleaner" the subnet.

The CBL index may be represented in Decibels (dB) or as CIDR suffix (*/xx).

Note: other spam researchers prefer to use a percentage of IPs
that are listed in a subnet. Using percentages is better suited for
"unclean" subnets because "clean" nets have significantly less than
1% of addresses listed.

Rationale

The CBL's full zone (data) is available publicly via rsync for
download, you are encouraged to register for it - see http://cbl.abuseat.org for more detail.

The CBL Index is a reasonably good tool for
getting estimates of subnet "outgoing spam
reputation".

The CBL Index should be treated with caution -
subnets often contain IPs with radically different purposes.
Assuming all IPs within a subnet represent the same risk/reputation
is potentially dangerous.

The CBL Index may be used for estimation of
overall anti-spam performance of ISP or AS operator.